1. Field of the Invention
The present invention relates to a light-emitting material for an organic electroluminescence device (to be sometimes referred to as "EL device" hereinafter) used as a flat light source or a flat display device, and an EL device having a high brightness.
2. Description of the Related Art
An EL device using an organic substance is overwhelmingly expected to be usable as a solid light-emitting inexpensive large-screen, a full-color display device, and other devices are being developed in many ways. Generally, an EL device is composed of a light-emitting layer and a pair of mutually opposite electrodes sandwiching the light-emitting layer. Light emission is caused by the following phenomenon. When an electric field is applied to these two electrodes, the cathode injects electrons into the light-emitting layer and the anode injects holes into the light-emitting layer. When the electrons recombine with the holes in the light-emitting layer, their energy level shifts to a valence bond band which causes them to release energy as fluorescent light.
As compared with inorganic EL devices, conventional organic EL devices require high voltage, and their light emission brightness and light emission efficiency are low. Further, conventional organic EL devices deteriorate in properties to a great extent, and no organic EL device has been put to practical use.
There has been recently proposed an organic EL device which is produced by laminating a thin film containing an organic compound having a fluorescent quantum effect of emitting light at a low voltage (as low as less than 10 V) which has attracted attention (Appl. Phy. Lett., Vol. 51, page 913, 1987).
The above organic EL device has a light-emitting layer containing a metal chelate complex and a hole-injecting layer containing an amine-based compound, and emits green light having a high brightness. The above organic EL device achieves nearly practically usable performance, since it accomplishes a brightness of thousands cd/m.sup.2 and a maximum light emission efficiency of 1.5 lm/W when a direct current voltage of 6 or 7 V is applied.
However, conventional organic EL devices including the above organic EL device are not yet sufficient in brightness thereby needing improvements in brightness to some extent. Further, their big problem is that they are unstable during continuous light emission operation over a long period of time. It is because, for example, the metal chelate complex such as tris(8-hydroxyquinolinate)aluminum complex is chemically unstable when light is emitted by the application of an electric field and because the metal chelate complex is poor in adhesion to a cathode so that the organic EL device deteriorates immensely in a short time during light-emitting operation. Therefore, in developing an organic EL device which has a higher light emission brightness and high light emission efficiency and which has excellent stability in continuous operation over a long period time, it is desirable to develop a light-emitting material having excellent light-emitting capability and durability.